Describingquantifying Emissions

Humans have observed and written about light-emitting insects for more than 2 millennia, but early in the 20th century they began to give careful scientific attention to different colors and forms of firefly emissions, noting glows, flashes, flickers, tremulations, scintillations, and so on, and they borrowed descriptive terms from other senses, such as

FIGURE 2 Flash patterns of fireflies as displayed for examination and measurement from electronic (frequency-modulated) field recordings, as described in the text, with standard display convention: horizontal axis, time; vertical axis, relative intensity (not total photon flux, which would be virtually impossible to measure from the entire surface of a flying light organ, but intensity change through time as detected from a single position in space, as visible to another firefly). Bars show time scale in seconds; horizonal grid in A, 0.02 s. (A) Nearly symmetrical flash of Photuris salina, photographed from the CRT screen of a storage oscilloscope. (B) Same flash scanned from ink tracing of a chart recorder. (C) Crescendo flash of Photuris unnamed species "D", with subliminal (to human eyes) modulations at the light organ. (D) Flash pattern of a Colombian Photinus, with subliminal modulations. (E) Crescendo flash of Photuris cinctipennis. (F) Visibly flickering flash pattern of a New Guinea Luciola species.

FIGURE 2 Flash patterns of fireflies as displayed for examination and measurement from electronic (frequency-modulated) field recordings, as described in the text, with standard display convention: horizontal axis, time; vertical axis, relative intensity (not total photon flux, which would be virtually impossible to measure from the entire surface of a flying light organ, but intensity change through time as detected from a single position in space, as visible to another firefly). Bars show time scale in seconds; horizonal grid in A, 0.02 s. (A) Nearly symmetrical flash of Photuris salina, photographed from the CRT screen of a storage oscilloscope. (B) Same flash scanned from ink tracing of a chart recorder. (C) Crescendo flash of Photuris unnamed species "D", with subliminal (to human eyes) modulations at the light organ. (D) Flash pattern of a Colombian Photinus, with subliminal modulations. (E) Crescendo flash of Photuris cinctipennis. (F) Visibly flickering flash pattern of a New Guinea Luciola species.

"crescendo" and "ramp" (Fig. 2E)—both describing a flash that begins dimly and gradually rises in intensity.

The simplest form of light emission is a glow of indeterminate length, as produced by fungus gnats, some Collembola, beetle larvae, adult fire beetles (elaters), phengodid beetle larvae and adult females, and lampyrid glowworm firefly females. A useful description of glows requires only a statement of apparent brightness (distance visible) and, cautiously, apparent color. In contrast, an adequate description of many adult lightningbug emissions often requires a chart, such as first published by Frank McDermott in 1914, with relative-intensity/time on the axes and notes of variations among flash patterns, from pattern to pattern, and at different ambient temperatures.

In the 1930s electronic technology, with photocells, string galvanometers, and ocillographs, made it possible to electronically chart firefly flashes in the laboratory. A generation later photomultiplier-tube systems permitted recording of the flashes of fireflies flying in the field. With today's miniaturized solid-state, digital circuits, detectors, and tape recorders flash detecting/recording systems can be hand-held. The flash patterns shown in Fig. 2 were all recorded in flight in the field, with a photomultiplier tube as detector, whose output was frequency modulated (fm) to encode intensity information; the fm conversions were recorded on magnetic tape. In the laboratory the recorded fm patterns were demodulated and fed into an oscilloscope (Fig. 2A) and chart recorder (Figs. 2B to 2F), for display and measurement. Video cameras add yet another dimension to observation and flashing behavior analyses.

There is no evidence that bioluminescent insects make use of color discrimination—it should be noted that the color a human observer perceives in the field often errs because of the intensity of the luminescence, its background (sky, street) lighting, and the degree of dark adaptation of the viewer's eyes. For example, dim green light may appear white because color vision apparatus (cone vision) is not stimulated, and the yellow flashes of roadside Photinus pyralis may appear green when under a sodium-vapor streetlight.